docs: def get_routes(
      feed: "Feed", date: Optional[str] = None, time: Optional[str] = None
  ) -> DataFrame:
      """
      Return a subset of ``feed.routes``
  
      Parameters
      -----------
      feed : Feed
      date : string
          YYYYMMDD date string restricting routes to only those active on
          the date
      time : string
          HH:MM:SS time string, possibly with HH > 23, restricting routes
          to only those active during the time
  
      Returns
      -------
      DataFrame
          A subset of ``feed.routes``
  
      Notes
      -----
      Assume the following feed attributes are not ``None``:
  
      - ``feed.routes``
      - Those used in :func:`.trips.get_trips`.
  
      """
      if date is None:
          return feed.routes.copy()
  
      trips = feed.get_trips(date, time)
      R = trips["route_id"].unique()
      return feed.routes[feed.routes["route_id"].isin(R)]